feat(taskflow): add core task API, storage persistence, csv export, stats page, and test coverage

tests/test_repository_atomicity.py

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+from __future__ import annotations
+
+import json
+from datetime import UTC, datetime
+from pathlib import Path
+from uuid import uuid4
+
+import pytest
+
+from app.storage import DATA_FORMAT_VERSION, StoredTask
+from app.storage.repository import JsonFileTaskRepository
+
+
+def make_task(title: str = "Task") -> StoredTask:
+    return StoredTask(
+        id=uuid4(),
+        title=title,
+        created_at=datetime.now(UTC),
+        started_at=None,
+        done_at=None,
+    )
+
+
+def test_interrupted_save_before_rename_keeps_original_data_intact(tmp_path, monkeypatch):
+    file_path = tmp_path / "tasks.json"
+    repo = JsonFileTaskRepository(file_path)
+    original_task = make_task("Original")
+    repo.create_task(original_task)
+
+    replacement_called = False
+    real_replace = Path.replace
+
+    def failing_replace(self: Path, target: Path):
+        nonlocal replacement_called
+        replacement_called = True
+        raise RuntimeError("simulated crash before rename")
+
+    monkeypatch.setattr(Path, "replace", failing_replace)
+
+    with pytest.raises(RuntimeError, match="simulated crash before rename"):
+        repo.create_task(make_task("New task"))
+
+    assert replacement_called is True
+
+    reloaded_repo = JsonFileTaskRepository(file_path)
+    assert reloaded_repo.list_tasks() == [original_task]
+
+    tmp_file = tmp_path / "tasks.json.tmp"
+    assert tmp_file.exists()
+
+    monkeypatch.setattr(Path, "replace", real_replace)
+
+
+def test_normal_save_replaces_file_with_complete_payload(tmp_path):
+    file_path = tmp_path / "tasks.json"
+    repo = JsonFileTaskRepository(file_path)
+    task_1 = make_task("One")
+    task_2 = make_task("Two")
+
+    repo.create_task(task_1)
+    repo.create_task(task_2)
+
+    payload = json.loads(file_path.read_text(encoding="utf-8"))
+
+    assert payload["version"] == DATA_FORMAT_VERSION
+    assert len(payload["tasks"]) == 2
+    assert {item["id"] for item in payload["tasks"]} == {str(task_1.id), str(task_2.id)}
+
+
+def test_stale_tmp_file_does_not_break_startup(tmp_path):
+    file_path = tmp_path / "tasks.json"
+    repo_1 = JsonFileTaskRepository(file_path)
+    task = make_task("Persisted")
+    repo_1.create_task(task)
+
+    stale_tmp = tmp_path / "tasks.json.tmp"
+    stale_tmp.write_text('{"version": 999, "tasks": "broken"}', encoding="utf-8")
+
+    repo_2 = JsonFileTaskRepository(file_path)
+
+    assert repo_2.list_tasks() == [task]
+    assert stale_tmp.exists()
+